<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
</head>
<body>
<style>
    div{width: 50px;height: 50px;border: 1px solid;}
</style>
<div></div>
<p><button id="btn">change</button></p>
<script>
    var btn=document.getElementById('btn');
    var div=document.getElementsByTagName('div')[0];
    btn.onclick=function(){
        div.style.border='double red';
    }
</script>
</body>
</html>